1. Field of the Invention
The present invention relates generally to power factor correction. More particularly, the present invention concerns high speed power factor correction in devices such as regulated power supplies, and methods for operating such devices.
2. Description of Related Art
Electric power is often provided as alternating-current (“AC”) power in the form of time-varying currents and voltages. Typically, the AC power is furnished by providing a supply voltage that varies over time at a fundamental supply frequency fAC. In North America, for example, AC power is typically provided at a supply voltage having a fundamental supply frequency fAC of 60 Hertz. In European countries, the fundamental supply frequency fAC is typically 50 Hertz.
FIG. 1 illustrates a simplified diagram of a system 100 for supplying AC power to a remote load 120. The load 120 may for example be a computer, a television, a household appliance, or any other electronic device which requires power to operate.
A power supply source 110, which may be a generator at the location of a utility company, provides power by generating an AC supply voltage VAC(t) at a fundamental supply frequency fAC such as 60 Hertz. The supply voltage VAC(t) from the power supply source 110 is then distributed to the load 120 via a transmission line of a power distribution system 140. Although not illustrated in the simplified diagram of FIG. 1, the power distribution system 140 may include transformers and other components utilized in the distribution of power.
The supply voltage VAC(t) is delivered to the load 120 to induce a load current ILD(t) to flow between the power supply source 110 and the load 120 via the power distribution system 140, thereby delivering power to the load 120. The power generated by the power supply source 110 and distributed over the power distribution system 140 is the ‘apparent power’ delivered to the load 120. In comparison, the portion of the apparent power that, when averaged over time, results in a net transfer of energy into the load 120 is the ‘actual power’ consumed by the load 120. The actual power may for example be consumed by the load 120 by the conversion of the electrical energy into non-electrical energy such as heat, light or mechanical energy.
Ideally, all of the apparent power delivered to the load 120 is consumed as actual power, so that power which has been generated and distributed to load 120 is not wasted. However, typically the actual power is less than the apparent power. In other words, the power consumed by the load 120 is less than the power that must be generated and delivered to the load 120.
Power factor is defined as the ratio of the actual power to the apparent power, and is a dimensionless number between 0 and 1. The value of the power factor is thus a measure of the power consumption efficiency of the load 120. When all of the apparent power is consumed as actual power by the load 120, the power factor is 1. A lower power factor for the load 120 will require more apparent power, and thus draw more load current ILD(t), for the same actual power consumed. Although not utilized by the load 120, this higher apparent power in the form of higher load current ILD(t) still must be generated by the power supply source 110 and distributed over the power distribution system 140. As such, this higher apparent power is subject to losses in the power generation and distribution processes. This places a heavy stress on the power supply source 110 and the power distribution system 140, and may require more expensive generation and distribution equipment. It is therefore desirable for the power factor of the load 120 to be as close to 1 as possible.
The power factor of the load 120 is dependent on the time-varying relationship between the supply voltage VAC(t) and the load current ILD(t), which in turn depends on the electrical characteristics of the load 120. Ideally, the load 120 emulates the electrical characteristics of a resistor. In such a case, the supply voltage VAC(t) and the load current ILD(t) are directly proportional and change polarity at the same instant in each cycle of the waveform, as shown in FIG. 2A. As a result, power flows in a single direction from the power supply source 110 into the load 120 at each instance in time, such that all of the apparent power is consumed as actual power. This results in a power factor of 1 for the load 120.
Typically, however, the load 120 is not composed entirely of resistive elements. Instead, the load 120 may include components which can cause the power factor to be less than 1. These components may include reactive components such as capacitors or inductors which temporally store a portion of the apparent power as energy in electric and magnetic fields. Rather than being consumed as actual power, this stored energy can then be returned back to the power supply source 110 a fraction of a second after it is stored. In other words, these reactive components can result in power flowing both from the power supply source 110 to the load 120, and from the load 120 to the power supply source 110. This returned power is non-productive power which, although not consumed by the load 120, must still be generated by the power supply source 110 and distributed over the power distribution system 140. In such a case, the apparent power is greater than the actual power, resulting in a power factor less than 1.
Non-linear components within the load 120 which interrupt or otherwise distort the waveform of the load current ILD(t) can also cause the power factor to be less than 1. For example, the load 120 may consist of an electronic device such as a computer or household appliance which requires direct-current (“DC”) power to operate. In such a case, a power supply circuit within the load 120 converts the AC power provided by the power supply source 110 into DC power. The power supply circuit typically converts the AC power into DC power using a rectifier circuit which includes diodes. The diodes in the rectifier circuit can result in the load 120 drawing a high load current ILD(t) only at the peaks of the supply voltage VAC(t), and drawing substantially zero load current ILD(t) at other instances in time. This results in the load current ILD(t) having a highly non-sinusoidal waveform as shown in FIG. 2B. The high peak currents in the load current ILD(t) result in significant power loss within the power distribution system 140 and place a heavy stress on the power supply source 110 and the power distribution system 140.
The distorted load current ILD(t) also results in the load current ILD(t) having higher frequency components at integer multiples of the fundamental supply frequency fAC. For example, if the fundamental supply frequency fAC is 60 Hertz, the distorted load current ILD(t) can include components at 60 Hertz, 120 Hertz, 180 Hertz, 240 Hertz, etc.
The component of the load current ILD(t) at the frequency fAC is referred to herein as the fundamental component ILD0(t). A component of the load current ILD(t) at frequency (m+1)fac, where m is a positive integer greater than or equal to 1, is referred to herein as the mth overtone component ILDm(t). The load current ILD(t) is therefore a superposition of the fundamental and overtone components. This can be represented mathematically as:
                                          I            LD                    ⁡                      (            t            )                          =                                            I                              LD                ⁢                                                                  ⁢                0                                      ⁡                          (              t              )                                +                                    ∑                              m                =                1                            M                        ⁢                                          I                LDm                            ⁡                              (                t                )                                                                        Eq        .                                  ⁢                  (          1          )                    where M is the total number of overtone components ILDm(t) within the load current ILD(t). The value Im at a given time to is the amplitude of the overtone component ILDm(t=t0) at time t=t0.
Although present within the load current ILD(t), the overtone components ILDm(t) are not consumed by the load 120 as actual power. In other words, the power within each of the overtone components ILDm(t) is non-productive power which, although not consumed by the load 120, must still be generated by the power supply source 110 and distributed over the power distribution system 140. As such, the presence of these overtone components ILDm(t) in the load current ILD(t) results in an apparent power greater than the actual power, thereby resulting in a power factor less than 1.
A power factor correction (PFC) circuit may be implemented within the load 120 to improve the power factor. The PFC circuit regulates the load current ILD(t) in an attempt to make the shape of the load current ILD(t) match the sinusoidal waveform of the supply voltage VAC(t). In doing so, the PFC circuit attempts to remove or suppress the overtone components ILDm(t) and obtain a power factor of 1.
The PFC circuit may control the shape of the load current ILD(t) via an adaptive feedback loop. The adaptive feedback loop is used to adjust the parameters of a control signal based on a measurement of the values of the overtone components present in the load current ILD(t). The control signal is then utilized to regulate the load current ILD(t) drawn by the load 120 such that the load current ILD(t) is directly proportional to, and in phase with, the supply voltage VAC(t). See, for example, U.S. Pat. No. 7,719,862, the disclosure of which is incorporated by reference herein.
An adaptive feedback loop can provide excellent steady-state power factor correction performance, thereby enabling efficient power consumption by the load 120 and reducing the stress on the power supply source 110 and the power distribution system 140. However, the transient response performance due to the delay introduced by the adaptive feedback loop continues to limit the overall power factor correction performance.
Transient events can occur in the system 100 when the electrical characteristics of the load 120 suddenly change. This can occur for example if the load 120 is turned on or off by a user. Transient events may also occur in the form of power supply source 110 glitches or surges. Due to the delay introduced into the power factor correction process by the adaptive feedback loop, the PFC circuit may not be able to adjust the load current ILD(t) quickly enough to maintain a power factor correction at or near 1. For example, due to the delay, the control signal not be adjusted until one-half cycle of the supply voltage VAC(t) or longer following a transient event. As a result, these transient events distort the load current ILD(t) and result in overtones components ILDm(t). This in turn causes the power factor of the load 120 to temporarily decrease, consequently decreasing the power consumption efficiency of the load 120 and increasing the stress on the power generation and distribution processes.
It is therefore desirable to provide systems and methods for high speed power factor correction that address the performance limitations associated with changes in the operating conditions of a load or other transient events.